Composite chromatographic article

ABSTRACT

A composite chromatographic article comprising: (a) a polytetrafluoroethylene fibril matrix, and 
     (b) non-swellable sorptive particles enmeshed in said matrix, the ratio of non-swellable sorptive particles to polytetrafluoroethylene being in the range of 19:1 to 4:1 by weight, said composite article having a net surface energy in the range of 20 to 300 milliNewtons per meter.

This is a division of application Ser. No. 07/137,811 filed Dec. 28,1987, now U.S. Pat. No. 4,810,381.

FIELD OF THE INVENTION

This invention relates to articles which are composite structures and amethod therefore, the articles comprising a polytetrafluoroethylene(PTFE) fibril matrix in which is enmeshed non-swellable particulate. Inanother aspect, a method of using the composite structures aschromatographic articles are disclosed.

BACKGROUND OF THE INVENTION

Chromatographic processes are known in the art. They provide a means ofseparating and analyzing mixtures of solutions by selective adsorptionon materials such as nylon, alumina, and silica. The process is based ondifferences in the distribution ratios of the components of mixturesbetween a mutually immiscible mobile and a fixed stationary phase. Inparticular, there are formed isolated spots or bands which can beseparated mechanically and further examined. In thin layerchromatography, it is known to use thin films, such as silica mixed witha binder (e.g. calcium sulfate) adhered to glass for the separatingvehicle.

U.S. Pat. No. 4,153,661 discloses a method of making apolytetrafluoroethylene composite sheet comprising a PTFE matrix withparticulate material, which is substantially insoluble in water,dispersed therein. The resulting sheet is extremely pliable, akin to doeskin. It is said to be useful as an electronic insulator or asemipermeable membrane.

U.S. Pat. No. 4,373,519 discloses a composite wound dressing comprisinga PTFE matrix with water-swellable hydrophilic absorptive particlesenmeshed in the matrix, and, optionally, a partially occlusive filmcoated on one surface of the matrix. It is disclosed that theparticulate material can account for from 40 to 90% by weight of thetotal composition, of which up to 50% can be inert property modifierparticles. Examples of property modifier particles include silica,kaolin, talc, bentonite, vermiculite, etc. The sheets are described asconformable and chamois-like.

U.S. Pat. Nos. 4,565,663 and 4,460,642, which are related to U.S. Pat.No. 4,373,519 (a division of a continuation-in-part application and acontinuation-in-part, respectively) disclose water-swellable compositesheets having a PTFE matrix in which water-swellable hydrophilicabsorptive particles are enmeshed. As in U.S. Pat. No. 4,373,519 thewater-swellable particulate can account for from 40 to 90% by weight ofthe total composition, of which up to 50% by weight can be inertproperty modifier particles, e.g. silica. The sheets are described asconformable and chamois-like. It is disclosed that they can be used aschromatographic materials. It is also disclosed that certainwater-swellable cation exchange resins can be used as particulate inchromatographic supports.

SUMMARY OF THE INVENTION

In contrast to the teachings of the prior art, it has been found thatwater swellable particles which undergo dimensional changes areundesirable in the chromatographic process. It has been found thatnon-swellable sorptive particles, rather than swellable particles areespecially useful and provide a desirable sorbent in chromatographicprocesses.

Briefly, the present invention provides a composite chromatographicarticle comprising:

(a) a polytetrafluoroethylene (PTFE) fibril matrix, and

(b) non-swellable sorptive particles enmeshed in said matrix, the ratioof non-swellable sorptive particles to PTFE being in the range of 19:1to 4:1 by weight, said composite article having a net surface energy inthe range of 20 to 300 milliNewtons per meter.

In another aspect, the present invention provides a method for providingfibrillated, semi-rigid, PTFE composite sheets havingchromatographically active non-swellable sorptive particles enmeshed andevenly distributed therein. These materials can be prepared fromchromatographically active non-swellable sorptive particles and a PTFEemulsion via a variation of the work intensive procedure described inU.S. Pat. No. 4,153,661, which procedure is incorporated herein byreference. Even distribution of particulate in the PTFE matrix does notallow for channeling of solutions flowing therethrough.

The chromatographic articles of the invention are useful in chemical andbiochemical separations and analyses.

In a further aspect, the present invention provides a method forchromatographic separation and analysis using the composite articledisclosed herein.

In this application:

"matrix" means an open-structured entangled mass of microfibers;

"hydrophobic particles" mean particles with low surface polarity, i.e.in the range of 0.1-0.5;

"semi-rigid" means flexible, dimensionally stable, and nonconformable;creasing results in cracking;

"ceramic" means nonmetallic, inorganic materials;

"direct phase system" means a more polar stationary phase with a lesspolar moving phase;

"reverse phase system" means a less polar stationary phase with a morepolar moving phase;

"non-swellable particulate" means particulate having a change in volume,

wherein change in volume=V_(g) -V_(o) /V_(o), of less then 0.5,preferably less than 0.1, most preferably less than 0.01, where V_(g) isthe volume of the particulate when swollen and V_(o) is the volume ofthe dry particulate.

"particles" or "particulate" means fibers of diameter 1 to 100micrometers, with a length to diameter ratio of 1 to 20, in addition toparticles as defined below;

"net surface energy" means the sum of polar and non-polar surfacetensions;

"self-supporting" means that no rigid backing support is needed for thearticle; and

"sorbent" or "sorptive" means capable of taking up and holding by eitherabsorption or adsorption.

Heretofore, the chromatographer or separation scientist skilled in theart selected a chromatographic sorbent which operated in either adirect, sorbent phase mode or in a reverse phase mode, or prepared anaggregation thereof, depending on the nature of the material to beseparated and/or purified. The aggregation of sorbent particles thenbecome an integral combination within the PTFE matrix.

In contrast, the present invention teaches chromatographic articleswhich can be operated concomitantly in a combination of both the directand the reverse phase modes. Dictation of these modes is determined andcontrolled by the ratio of PTFE matrix and direct phase sorbent that areintimately present in fabricated chromatographic articles of thisinvention.

BRIEF DESCRIPTION OF THE DRAWING

In the accompanying Drawing:

FIG. 1 is a cross-sectional view, greatly enlarged, of a compositearticle of the present invention;

FIG. 2 is a top plan view of one embodiment of the invention which is acircular composite chromatographic disk that has been used for ananalytical separation;

FIG. 3 is a top plan view of a variation of the embodiment of FIG. 2which has been used for an analytical separation;

FIG. 4 is a top plan view of another embodiment of the invention inwhich a chromatographic strip has been used for an analyticalseparation.

DETAILED DESCRIPTION OF THE DRAWING

FIG. 1 shows one embodiment of a composite article 10 according to thepresent invention having matrix 12 of PTFE fibrils 14 in which areenmeshed active, sorptive, non-swellable particles 16. Support 18, shownin broken lines, is optionally included in the composite article.

FIG. 2 shows self-supporting chromatographic disk 20 having a PTFE tosilica ratio of 10.90. In one embodiment, spots 22, 24, 26, and 28 of asolution of one or three components have been placed, separated fromeach other, in a circular configuration and then disk 20 has beensubjected to spinning. With continuous spinning (rotating), solvent iswicked onto the disk just inside of the spotted circular configuration.While spinning continues, solvent is continuously added and is forcedoutward by centrifugal forces. A separation occurs into spots 25, 23,and 27. Spots 24, 28, and 26 are solutions of a single component which,after spinning, show migration of the component to spots 25, 23, and 27,respectively. Spots 22 represent a solution of a mixture of threecomponents which, after spinning, show migration of the resolved andseparated components as spots 25, 23, and 27. When spots 22 were a dyemixture of Methyl Yellow, Sudan Red, and Indophenol Blue in organicsolvent (e.g. toluene), the separation showed Indophenol Blue spots 25,Sudan Red spots 23, and Methyl Yellow spots 27. When spot 24 was asolution of Indophenol Blue in organic solvent, spot 25 was a blue spot.When spot 26 was a solution of Methyl Yellow in organic solvent, spot 27was a yellow spot. When spot 28 was a solution of Sudan Red in organicsolvent, spot 23 was a red spot.

FIG. 3 shows a variation of the embodiment of FIG. 2. Self-supportingchromatographic disk 30 having a PTFE/silica ratio of 10/90 is allowedto spin while a source of organic solution (i.e., sample) containingcomponents to be separated comes in contact with disk 30. This formscircular zone 32 of deposited sample. This process is well known in theart and is called radial chromatography. While spinning continuessolvent is added to sample 32 and is forced outward by centrifugalforces A separation (resolution of the mixture) occurs into circularcomponent bands 33, 35, and 37. When the three components are the samedyes as used in the embodiment of FIG. 2 (dissolved in toluene), bandsseparated as Indophenol Blue band 33, Sudan Red band 35, and MethylYellow band 37.

FIG. 4 shows another embodiment of the invention. Thin (0.75 mm)chromatographic strip 40, having a PTFE/silica ratio of 20/80, has beeninscribed with spot 42, a solution of three components. The strip wasallowed to be in contact with a solvent (e.g. 0.5% methanol in methylenechloride) for a time sufficient to allow the solvent to be wicked upstrip 40 by capillary action. This resulted in the separation ofcomponents into spots 43, 45, and 47. When the components were the samethree dyes as disclosed in FIG. 2, spot 43 separated as Indophenol Blue,spot 45 separated as Sudan Red, and spot 47 separated as Methyl Yellow.

DETAILED DESCRIPTION

The particulate material (which can be one material or a combination ofmaterials) useful in the present invention is non-swellable in aqueousand organic media and is substantially insoluble in water or the elutionsolvent. Not more than 1.0 gram of particulate will dissolve in 100 g.of aqueous media or elution solvent into which particulate is mixed at20° C. The particulate material can be an organic compound, a polymer,or an inorganic oxide such as silica, alumina, titania, zirconia, andother ceramics, or it can be ion exchange or chelating particles.Preferred particulate material are silica and zirconia, with silicabeing particularly preferred because of the ease in bonding a variety ofhydrophobic and semi-hydrophobic coatings onto its surface and becausethey are commercially available.

Suitable particles for the purposes of this invention include anyparticle which can be coated with insoluble, non-swellable sorbentmaterial or the surface (external and/or internal) of which can bederivatized to provide a coating of insoluble, non-swellable sorbentmaterial. Preferred supports for such coatings include inorganic oxideparticles, most preferably silica particles. The insoluble,non-swellable sorbent coatings generally have a thickness in the rangeof one molecular monolayer to about 300 micrometers. Such particleshaving coated surfaces are well known in the art, see, for example,Snyder and Kirkland, "Introduction to Modern Liquid Chromatography", 2dEd., John Wiley & Sons, Inc. (1979) and H. Figge et al., "Journal ofChromatography" 351 (1986) 393-408. The coatings can be mechanicallyapplied by insitu crosslinking of polymers or the coatings can befunctional groups covalently bonded to the surface of the particles.Many such coated particles are commercially available (e.g., C₁₈ bondedphase silica, Alltech, Deerfield, IL).

Coatings which can be applied to silica particulate can be either thinmechanical coatings of insoluble, non-swellable polymers such ascrosslinked silicones, polybutadienes, etc. or covalently bonded organicgroups such as aliphatic groups of varying chain length (e.g., C₂, C₈,and C₁₈) and aliphatic and aromatic groups containing amine, nitrile,hydroxyl, chiral, and other functionalities which alter the polarity ofthe coating. The silica, or other support particle, in this case actsprimarily as a carrier for the organic coatings and the particles arenon-swellable. The variation in chemical composition of the coatingsprovides selectivity in molecular separations and polarity.

The particulate material may have a spherical shape, a regular shape oran irregular shape. Particulate material which has been found useful inthe invention has an apparent size within the range of 0.1 to about 600micrometers, preferably in the range of 1 to 100 micrometers. It hasbeen found advantageous in some instances to employ particulatematerials in two or more particle size ranges falling within the broadrange. As an example, particles having an average size in the range of0.1-30 micrometers having chromatographic activity may be employed incombination with particles having an average size in the range 1 to 250micrometers acting as a property modifier.

Some particle size reduction may take place during the high shear mixingand the calendering operations, depending upon the friability of theparticulate material. Thus, while the particulate material initially maybe rather large, it may ultimately be reduced to a finer size in thefinal product.

Particles useful in the present invention have water sorptive capacityless than 10% by weight, preferably less than 1% by weight. As notedabove, particles which undergo dimensional changes due to waterswellability are less desirable. In view of the teachings of U.S. Pats.4,565,663 and 4,460,642 it is surprising that hydrophobic particles andother non-swellable particles enmeshed in PTFE provide superiorchromatographic articles compared to water-swellable hydrophilicparticles enmeshed in PTFE.

As described in the method of U.S. Pat. No. 4,153,661, the activesorbent particles useful in the present invention can be pre-mixed witha property modifier which can function, for example, as processing aid.Representative non-swellable property modifiers (some of which may besoluble in water) can be coated particles (e.g., cation exchange resins)calcium carbonate, ammonium carbonate, kaolin, sugar, polyethylenes,polypropylenes, polyesters, polyamides, polyurethanes, polycarbonates,zeolites, chitin, vermiculite, clay, ceramics, ion exchange andchelating particles, and the like. These property modifier materials canbe present in an amount in the range of 0 to 28.99 parts per part ofPTFE, preferably 0 to 9.00 parts per part of PTFE, provided that thesorbent non-swellable particles plus property modifiers do not exceed 29parts particulate to 1 part PTFE. These ranges are desirable to achievea preferred tensile strength of at least 0.5 MegaPascal (MPa) in thecomposite structure.

Other non water-swellable property modifiers may be advantageously addedto the mixture of the PTFE aqueous dispersion and the primaryparticulate material to provide further improvement in or modificationof the composite films of the invention. For example, modifierparticulate can include chromatographically inactive materials such aslow surface area glass beads to act as property modifiers and processingaids. It is desirable from a surface energy standpoint to minimize thePTFE level and at times to alter the level of the active particulate.Coloring or fluorescesing particulate can be added at low levels (up to10 weight percent of particulate) to aid in visualizing samplecomponents to be separated. Chemically active particulate which indicatepH or acidity of the component bands can be useful for diagnosticpurposes.

A limited amount of water-swellable property modifiers (i.e., up to 30weight percent, preferably less than 25 weight percent, more preferablyless than 10 weight percent, and most preferably less than 1 weightpercent, of total particulate) can be useful as a processing aid.Representative swellable property modifiers include starch, chitosan,modified starches such as Sephadex™ and Sepharose™ (Pharmacia, Sweden),agarose, polymethacrylates, styrene-divinylbenzene copolymers,polyacrylamides, cellulosics, and coated particles (e.g., silica coatedwith a polyacrylamide). Water-swellable materials may be used as a thincoating on non-swellable particulate.

When the particulate is hydrophobic, the preferred method of manufactureof the article of the invention utilizes an emulsion of PTFE with amasking agent added to modify the hydrophobic particle surface/waterinteraction and allowing rapid wetting of the surface of the hydrophobicparticulate. Preferred masking agents are polar organic compounds suchas alcohols, amines, acids, etc. with the preferred group being alcoholsdue to their efficacious removability as by solvent extraction or dryingafter formation of the article.

Specifically, the PTFE composite sheet material of the invention isprepared by dry blending the particulate or combination of particulatesemployed until a uniform dispersion is obtained and adding a volume ofmasking agent up to approximately one half the volume of the blendedparticulate. The blending takes place along with sufficient lubricantwater to exceed the sorptive capacity of the particles. The aqueous PTFEdispersion is then blended with the particulate/masking agent mixture toform a mass having a putty-like or dough-like consistency. The sorptivecapacity of the solids of the mixture is noted to have been exceededwhen small amounts of water can no longer be incorporated into the masswithout separation. Care should be taken to ensure that the ratio ofwater to masking agent does not exceed 3:1. This condition should bemaintained throughout the entire mixing operation. The putty-like massis then subjected to intensive mixing at a temperature maintainedbetween about 50° C. and 100° C. for a time sufficient to cause initialfibrillation of the PTFE particles. Minimizing the mixing at thespecified temperature is essential in obtaining chromatographictransport properties.

Mixing times will typically vary from 0.2 to 2 minutes to obtain thenecessary initial fibrillation of the PTFE particles. Initialfibrillation causes partial disoriented fibrillation of a substantialportion of the PTFE particles.

Initial fibrillation will be noted to be at an optimum within 60 secondsafter the point when all components have been fully incorporatedtogether into a putty-like (dough like) consistency. Mixing beyond thispoint will produce a composite sheet of inferior chromatographicproperties.

The devices employed for obtaining the necessary intensive mixing arecommercially available intensive mixing devices which are sometimesreferred to as internal mixers, kneading mixers, double-blade batchmixers as well as intensive mixers and twin screw compounding mixers.The most popular mixer of this type is the sigma-blade or sigma-armmixer. Some commercially available mixers of this type are those soldunder the common designations Banbury mixer, Mogul mixer, C. W.Brabender Prep mixer and C. W. Brabender sigma blade mixer. Othersuitable intensive mixing devices may also be used.

The putty-like mass is then transferred to a calendering device where itis calendered between rolls maintained at about 50° C. to about 100° C.to cause additional fibrillation and consolidation of the PTFEparticles, while maintaining the water level of the mass at least at alevel of near the absorptive capacity of the solids, until sufficientfibrillation occurs to produce the desired chromatographic sheetmaterial. Preferably the calendering rolls are made of a rigid materialsuch as steel. A useful calendering device has a pair of rotatableopposed calendering rolls each of which may be heated and one of whichmay be adjusted toward the other to reduce the gap or nip between thetwo. Typically, the gap is adjusted to a setting of 10 millimeters forthe initial pass of the mass and, as calendering operations progress,the gap is reduced until adequate consolidation occurs. At the end ofthe initial calendering operation, the sheet is folded and then rotated90° to obtain biaxial fibrillation of the PTFE particles. Smallerrotational angles (e.g., 20° to less than 90°) may be preferred in somechromatographic applications to reduce calender biasing, i.e.,unidirectional fibrillation and orientation. Excessive calendering(generally more than two times) in thin layer chromatographic compositesreduces the solvent flow rate resulting in longer run times perseparation.

The calendered sheet is then dried under conditions which promote rapidwater evaporation yet will not cause damage to the composite sheet orany constituent therein. Preferably the drying is carried out at atemperature below 200° C. The preferred means of drying is by use of aforced air oven. The preferred drying temperature range is from 20° C.to about 70° C. The most convenient drying method involves suspendingthe composite sheet at room temperature for at least 24 hours. The timefor drying may vary depending upon the particular composition, someparticulate materials having a tendency to retain water more thanothers.

The chromatographic activity of particulate such as alumina or silicaused in the direct phase mode is adjustable by control of the watercontent. It is known in the art that the activity of alumina and silicacan be modified by addition of water. Selection of drying conditionsaffects the activity of these particles. Drying conditions must beindividually determined to obtain optimal separations of given samples.These conditions are available from particulate suppliers' literature,journal publications, and experimentation. Vacuum oven drying isrecommended in some applications. Typically, drying times will vary fromabout 1 hour to about 100 hours.

The resultant composite sheet has a tensile strength when measured by asuitable tensile testing device such as an Instron (Canton, Mass.)tensile testing device of at least 0.5 MPa. The resulting compositesheet has uniform porosity and a void volume of at least 30% of totalvolume.

The PTFE aqueous dispersion employed in producing the PTFE compositesheet of the invention is a milky-white aqueous suspension of minutePTFE particles. Typically, the PTFE aqueous dispersion will containabout 30% to about 70% by weight solids, the major portion of suchsolids being PTFE particles having a particle size in the range of about0.05 to about 0.5 microns. The commercially available PTFE aqueousdispersion may contain other ingredients, for example, surfactantmaterials and stabilizers which promote continued suspension of the PTFEparticles.

Such PTFE aqueous dispersions are presently commercially available fromDupont de Nemours Chemical Corp., for example, under the trade namesTeflon™ 30, Teflon™ 30B or Teflon™ 42. Teflon™ 30 and Teflon™ 30Bcontain about 59% to about 61% solids by weight which are for the mostpart 0.05 to 0.5 micrometer pTFE particles and from about 5.5% to about6.5% by weight (based on weight of PTFE resin) of non-ionic wettingagent, typically octylphenol polyoxyethylene or nonylphenolpolyoxyethylene. Teflon™ 42 contains about 32 to 35% by weight solidsand no wetting agent but has a surface layer of organic solvent toprevent evaporation. It is generally desirable to remove, by organicsolvent extraction, any residual surfactant or wetting agent afterformation of the article.

Silica is available from Aldrich Chemical Co. (Milwaukee, WI). Zirconiais available from Z. Tech Corporation (Bow, NH). Other inorganic oxidesare available (Aldrich Chemical Co.).

The present invention provides a novel composite structure and methodtherefore, the composite structure preferably being a uniformly porous,composite sheet comprised of non water-swellable sorptive particlesdistributed uniformly throughout a matrix formed of interentangled,fibrillated PTFE fibrils. In such a structure almost all of theparticles are separate one from another and each is isolated in a cagethat restrains the particle on all sides by a fibrillated mesh of PTFEmicrofibers. The preferred novel sheet of the invention has a thicknessin the range of 125 to 10,000 micrometers and has a tensile strength ofat least 0.5 MPa and even as high as 13.6 MPa. The article issubstantially uniformly porous, making it suited for use as achromatographic composite article which can be used as a singleself-supporting sheet or a combination of sheets to form a stack or as acomposite film adhered to a support such as glass, paper, metals, orpolymers.

The PTFE-particulate technology can be useful in a first mode whereinthe composite article cf the invention is used for preconcentration andisolation of certain materials for further analysis by high resolutioncolumn chromatography. In this mode, which is well known in the art,solvent and sample flow are introduced at an angle of 90 degrees to thesurface of the sheet. This is a conventional configuration and theseparation path length is equal to the thickness of the sheet. The pathlength can be increased by stacking additional layers but the individuallayers are not intimately bound together since the calendering operationis limited to a specific thickness. This mode is effective for one stepor multi step adsorption-desorption separations. This mode is effectiveusing reactive particulate such as non-swellable cation exchangematerials or sorptive particulate in the direct or reverse phase modes.We can expand the utility of this membrane mode by inclusion of manyother reactive particulates to carry out chemical and physical reactionsto be described. The article strongly adsorbs the component of interestonto the active (non-swellable) particulate in the composite andundesirable components are washed out with a first solvent. A stronger,generally more polar second solvent is then used to displace the desiredcomponent from the particulate allowing it to be recovered in a moreconcentrated and unified form. We found we could also form reactivemembranes choosing particulate for ion exchange, chelation,oxidation/reduction reactions, steric exclusion, catalysis, etc.

In a second mode, the flow is parallel to the surface or 0 degrees intothe edge of the sheet and the path length for the separation can beselected from the length of the material used and the ability totransport solvent by capillary action. Multiple, continuous sorption anddesorption steps are needed to obtain chromatrographic separations andrequire a minimum column length which is not practical to obtain bystacking disks of the composite in column configuration.

In the second mode, the separations and analysis is analogous to thinlayer or paper chromatography where solvents and sample components arealso transported through the media by capillary action. The compositecan be useful in a paper (PC) or thin layer chromatographic (TLC) modewhere the separations are obtained not through the composite at a 90degree mode but edgewise at a 0 degree mode.

It is believed that the migration rates through the composite article isproportional to the net surface energies of the PTFE filaments, thechromatographically active particulate such as silica, and a modifierparticulate. The small amount of PTFE appears to dominate these rates.This may be due to the construction wherein the active silica particlesdo not touch each other and the solvent mobility is dependent on the lowsurface energy PTFE fibrils. Electron beam treatment of the PTFE matrixwas investigated and increased the migration rates by 10%. In apreferred mode, using silica as particulate, a number of experimentswere performed varying the ratios from 95/5 to 80/20 (silica/PTFE) andwe found that the higher the silica content, the faster the rate ofsolvent and component migration.

The net surface energy of the composite article is the net weightedaverage of the surface energies of PTFE marix (E_(PTFE)), the activesorptive particulate (E_(part)), and modifying particulate (E_(mod)). Itis desirable that the net surface energy be in the range of 20 to 300milliNewtons per meter, preferably 50 to 300 mN/M. This providesoptimization of surface tension forces for solvent and solute transport.The net surface energy of a particulate is comprised of polar andnon-polar forces. Polarity is equal to the ratio of polar surfacetension to the total surface tension. Polarity of PTFE, Nylon 66, andsilica are calculated from surface tension data to be 0.10, 0.21, and0.38, respectively.

The composite articles of the present invention have high capacity forsample loading and can be very useful for preparatory or process scalechromatography. The migration rate can be increased dramatically usingradial chromatography wherein centrifical force is utilized to drive thesolvent through the chromatographic article. This process is well knownin the art. In the prior art chomatographic materials, higher amounts ofbinder are normally needed to hold the silica to the conventionalspinning glass plate, whereas in the present invention articles the PTFEmaterial needs no binder or supporting plate. In the prior art,particulates successfully adhered to glass plates have been limited tosilica and alumina. The present invention has a great advantage in thatvirtually any non-swellable organic or inorganic particulate can betrapped in the PTFE matrix for many chromatographic applications. Nopolar binder is required. The absence of the polar binder is ofparticular significance in reverse phase systems with non-swellablehydrophobic particulate.

The composite chromagraphic articles of the invention can be of anydesired size and shape. Preferably the articles can be sheet-likematerials which, for example, can be in disk or strip form. Coating thenon-swellable particulate with very thin (monolayer) materials orthicker materials provided by in-situ crosslinking of polymers orcovalently bonding functional molecules on the surface of theparticulate allows for the optimization of the both chromatographicselectivity and separation efficiency.

The composite articles have utility in a wide variety of separationswherein the choice of the particulate material is useful for sizecontrolled filtration or steric exclusion, for simple one step ormultistep adsorption-desorption separations of specific components, forimmobilization of reactive particulate to perform chemical orbiochemical reactions, for ion-exchange conversion and isolation ofcations and anions, for purification of materials, and forchromatographic separations and analyses in both passive and forced flowmodes, for hydrophobic reverse phase and direct phase chromatography.

Objects and advantages of this invention are further illustrated by thefollowing examples, but the particular materials and amounts thereofrecited in these examples, as well as other conditions and details,should not be construed to unduly limit this invention.

EXAMPLE 1

Method of making a 20/80 PTFE/hydroxylapatite composite article was asfollows:

Twenty grams of hydroxylapatite HTP grade (calcium phosphate availablefrom Bio Rad, Inc. of Richmond, Ca) was placed in a 100 ml beaker. Eightand 1/3 grams of polytetrafluoroethylene (PTFE) resin emulsion (Teflon™30B, Dupont, Inc , Wilmington, DE) was added stepwise in three portionswith intermittent vigorous stirring. Fifteen grams of water was thenadded stepwise in three portions with intermittent vigorous stirring.

After these ingredients had been thoroughly mixed, a semi-coherentmaterial was formed with enough physical integrity to allow the entirecontents to be removed from the beaker as a single mass. The above masswas passed through two rollers kept at 50° C. and spaced about 0.5 cmapart to give a strip of cohesive material of dimensions approximately15 cm×0.5 cm×5 cm. The resulting strip was folded to three thicknessesor a material having dimensions of 5 cm×1.5 cm×5 cm and then passedthrough the rollers after a 90° rotation from the previous pass. Thecyclic process of three-layer folding and re-rolling in the direction90° from the direction of the preceding pass was repeated a total of 10times to give a tough, strong, flat piece of material of dimensions 5cm×1.5 cm×5 cm. The material was then calendered along the long axisthrough a set of ten rollers which were spaced at successively smallerdistances apart to give a continuous ribbon of dimensions 8 cm×0.1 cm×80cm. The ribbon was folded to give a 8-layered piece of dimensions 8cm×0.8 cm×10 cm. The 8-layered piece was then calendered as before alongthe 10 cm axis (90° ) from the calendering direction used previously) togive a ribbon of dimensions 16 cm×0.08 cm×20 cm. By calendering usingvarying spaced rollers, different degrees of compaction of the masscould be obtained and various thicknesses of ribbon, as desired,realized. The calendered sheet of material was washed in a water bathand then allowed to dry in air for 48 hours.

Proteins (horse) were separated in a one step adsorption, three stepdesorption process with successfully higher ionic strength solutions. A25 millimeter, 50 micrometers thick disk with a PTFE to hydroxylapatiteratio of 20 to 80 was placed in vacuum filter holder and preconditionedby addition of 3 millimolar phosphate buffer at pH of 6.8. Flow rate was1.75 milliliters per minute per square centimeter. 50 microliter of asolution of Hemoglobin, Myoglobin, and Cytochrome C (Sigma ChemicalCorp., St. Louis, MO) in 3 millimolar buffer containing 0.01 weightpercent sodium azide was deposited onto the disk. In the firstdesorption step, 50 millimolar phosphate buffer solution effectivelyremoved the Hemoglobin protein. In the second step, 200 millimolarbuffer solution desorbed the Myoglobin protein and in the third step,500 millimolar buffer displaced the Cytochrome C. This separation wasreadily upgraded to longer path length modes where solvent wasintroduced through the lengthwise direction of the composite as in thethin layer embodiment, centrifugal force assisted, or with gradientelution pumping systems wherein the ionic strength or pH of the mobilesolvent phase was changed in a continuous rather than a stepwisefashion. A variety of active particulate in the polysacharide class suchas agarose, sepharose, cellulose, chitosan, etc. either native orderivatized, are useful in the adsorption, gel permeation or affinitychromatographic modes for biochemical type separation. Otherparticulates that can be used include polyacrylamides,polymethacrylates, and cross-linked copolymers such asstyrene-divinylbenzene copolymers are useful for a variety ofchromatographic separations.

EXAMPLE 2

Twenty grams of TLC grade silica (available from Aldrich Chemical Co.,Milwaukee, WI, was placed in a 100 ml beaker. 8.3 grams ofpolytetrafluoroethylene (PTFE) resin emulsion (Teflon 30B, Dupont) wasadded stepwise in three portions with intermittent vigorous stirring.Fifteen grams of water was then added stepwise in three portions withintermittent vigorous stirring. After formation of a putty-like mass,additional processing was performed according to the procedure ofExample 1.

Runs analogous to thin layer chromatography were carried out with thestandard material where the ratio of PTFE to silica was 20/80. Thisratio is generally chosen to impart tear resistance, rigidity, and otherphysical properties to the membrane. In these evaluations, a 225micrometer (15 mil) thick membrane was spotted with sample (a dyemixture of Methyl yellow, Sudan Red, and Indophenol Blue) as inconventional thin layer chromatography and the strip (1.5 cm wide, 12.5cm long) was suspended via a wire holder in a 50 ml graduated cylinder.Enough solvent (0.5% methanol in methylene chloride) was added tocontact the lower edge of the strip. The solvent wicks up the strip bycapillary action and the sample components are separated according totheir differences in partitioning coefficients between the movingsolvent front and the stationary absorptive particulate. Thosecomponents more strongly absorbed to the particulate move more slowlyand separations obtained indicated that the surface activity of theparticulate was not diminished by its inclusion in the PTFE web. We weresurprised to find that while separation of components was obtained, therate of solvent and solute migration was approximately 40 times slowerthan with a conventional TLC plate even though the material was 80percent silica. (Conventional TLC plate coatings are approximately 87percent silica with 13 percent CaSO₄ -(H₂ O)_(n) used to bind the silicato the glass plate.) Runs were then performed to define the effect ofratios of particulate to PTFE. Calendering parameters, and modifyingparticulate will be described later.

Using a similar procedure as described for the 20/80 articles, articleshaving a 10/90 ratio of PTFE to silica were prepared as follows:

Twenty grams of TLC grade silica (available from Aldrich Chemical Co.,Milwaukee, WI) was added to a 100 ml beaker. 3.7 grams ofpolytetrafluroethylene (PTFE) resin emulsion (Teflon 30B, Dupont) wasadded stepwise in two portions with intermittent vigorous stirring.Twenty grams of water were then added stepwise in four portions withintermittent vigorous stirring. After formation of a putty-like mass,additional processing was performed according to the procedure ofExample 1.

TABLE 1, below, gives data on elution time vs solvent front travel for acommercially available MERCK TLC plate, a 10/90 ratio of PTFE/silica(10A), and a 20/80 ratio of PTFE/silica (20A).

                  TABLE 1                                                         ______________________________________                                        Composition vs. Rate of Trend                                                 Point on  mm        Control    10A   20A                                      Time Curve                                                                              TRAVEL    (min)      (min) (min)                                    ______________________________________                                        2         5         0.16       0.75  1.50                                     3         10        0.47       1.83  4.16                                     4         15        0.93       3.50  9.00                                     5         20        1.66       5.75  15.16                                    6         25        2.50       8.42  23.16                                    7         30        3.50       11.50 32.16                                    8         35        4.75       15.16 42.00                                    9         40        5.93       18.42 50.50                                    10        45        7.16       21.50 58.66                                    11        50        8.66       24.50 67.00                                    ______________________________________                                    

The data of TABLE 1 show that the solvent front rate of travel forcomposites of the invention was less than rate of travel using aconventional TLC plate. The rate of travel for composite 10A containing10% PTFE was two to three times faster than composite 20A containing 20%PTFE.

Careful examination of the data shows that the differences in rate oftravel increase with increasing distances of solvent travel from theorigin. The distance of travel and time of separation chosen in practiceis dependent on the ability of the chromatographic system to separate orresolve the components in the sample mixture. The efficiency of thechromatographic system or its ability to separate the components in asample mixture is dependent on a number of factors. If the solvent flowrate is too high or too low, the resolution is degraded. Flow rates onthe commercial silica coated glass plate are dictated and fixed by thecapillary action contributions of the active particulate, the binderused to hold the particulate in place, and the glass or plastic plateused to support the particulate-binder media. In this invention, theflow rates are controllable by optimizing the composition of the matrixand the ratios of PTFE, active particulate, and modifying particulate.

A second factor affecting the resolution of the chromatographicseparation is the size and surface area of the active particulate. Ingeneral, the smaller the particle, the better the resolution. Particlesas small as 3 to 5 micrometers have been used in high resolutionchromatographic columns. These columns can be much shorter and yetdeliver the same resolution as longer columns with larger particles. Alimiting factor is the pressure drop and the difficulty in uniformpacking of the column. In this invention, the particle size can be assmall as 0.1 micron and therefore shorter separation paths are possible.

We also studied the method of making and calendering the membrane andfound that the more heavily calendered membranes had dramatically slowerrates. TABLE 2 illustrates the elution time data for a series of samples10A through 10D and a Merck control silica plate.

EXAMPLE 3

                  TABLE 2                                                         ______________________________________                                        Effect of Calendaring on Rate of Trend                                                      Control   Sample                                                                              Sample Sample                                                                              Sample                                   mm      Plate     10A   10B    10C   10D                                Sample                                                                              Travel  (min)     (min) (min)  (min) (min)                              ______________________________________                                        12            0         0     0      0     0                                  13     5      .17       1     .75    .75   1.5                                14    10      .37       1.5   2.0    2.0   4.0                                15    15      .75       2.25  4.0    4.5   11.0                               16    20      1.5       3.08  6.5    7.33  17.5                               17    25      2.2       5.5   9.25   11    26                                 18    30      3.08      7.5   13     15    36                                 19    35      4.03      10    16     20    43                                 20    40      5.0       13    20     24    51                                 21    45      6.08      16    24     28    58                                 22    50      7.0       18.5  28                                              23    55      8.33      21                                                    ______________________________________                                    

The data of TABLE 2 show the effect of method of making on thechromatographic properties of the composite. Samples 10A through 10D areidentical in composition, i.e., 10 percent PTFE-90 percent silica; butdiffer in the degrees of calendering. 10A, 10B, 10C, and 10D have beencalendered 1, 2, 3, 4 times respectively. The data show that calenderinggreatly increases the time required for the solvent front to travel agiven distance. Increased working of the PTFE increases the effect ofthe low surface energy of the PTFE on the net behavior of the composite.In this invention the diffusion of solvent by capillary action can becontrolled by the degree of calendering.

In a third embodiment, the separations and analysis obtained bycapillary action are assisted by centrifugal force as in radialchromatography. In prior art, the active chromatographic particulate ismixed with a binder such as starch or calcium sulfate and the mixture iscoated onto a circular glass disk. After drying and scraping the coatingto obtain a smooth uniform surface, the disk is rotated, typically at700 rpm, and the silica is prewetted with solvent. Solvent is "wicked"onto the disk near the center and is drawn through the chromatographicmedia to the outer edge by centrifical force. Sample is then wicked ontothe surface of the disk and appears as a continuous circle near thecenter of the disk. Solvent flow is then initiated and as separationoccurs, circles of increasing diameter appear for each component of thesample mixture. It is difficult to adhere the particulate to the platein this case and to date only silica and alumina are in general use.

In this invention, no binder, i.e., calcium sulfate, needed since theinert PTFE fibrils hold the particulate in place. Therefore virtuallyany organic or inorganic particulate can be incorporated into the matrixin disk form, greatly expanding the utility of the chomatographicseparation process.

Composites of 750 to 3750 micrometers (30 to 150 mils) in thickness havesufficient dimensional stability or rigidness to be rotated at 700 rpmwithout additional support. Thinner composites between 250 to 750micrometers (10 and 30 mils) can be supported with a rigid disk of lowsurface energy such a Teflon, certain plastics resistant to solventflow, and coated glass. The PTFE-particulate composite is not adhered tothe rigid disk which only serves to support it when it is not spinning.The low surface energy surface of the supporting disk minimizes solventstreaking. Sample can be introduced to the disk (FIG. 3) either as acontinuous circular line (#32) by the wicking operation as described orcan be added as discreet spots to the stationary disk as shown in FIG.2. We found that when thicker disks are used for larger scaleseparations, it is advantageous to inject the sample into the diskrather than to spot it onto the surface. Solvent flow is then initiatedand components of the sample mixture are separated into bands as in thinlayer chromatography but more rapidly due to the forced flow provided bythe centrifical force on the spinning disk. In FIG. 3, the threecompounds separated are Indophenol Blue, Sudan Red, and Methyl Yellow.When the samples are injected into the disk as shown in FIG. 2, it ispossible to inject up to 32 samples or reference compounds on a 6 inchdiameter disk. This allows the analyst to compare the migration rates ofknown compounds with those rates observed in the sample mixture. Thematerial is easily cut for isolation purposes. For example a cork borercan be used to remove any spot on the disk. Extracting the spot with asolvent allows the analyst to recover the purified material forsubsequent tests without the contamination from silica or binderparticulate.

Alternatively, the solvent flow can be continued, washing the separatedbands of sample components from the outer edge of the disk intoindividual collection vessels as is known in the literature. The diskscan be thoroughly washed with solvent using an appropriate vacuumfiltration funnel and oven dried for reuse with new samples.

Various modifications and alterations of this invention will becomeapparent to those skilled in the art without departing from the scopeand spirit of this invention, and it should be understood that thisinvention is not to be unduly limited to the illustrative embodimentsset forth herein

We claim:
 1. A method of chromatographic separation comprising the stepsof:(a) spotting a solution containing separable components in a circularconfiguration onto a rotatable chromatographic sheet-like articlewherein said chromatographic sheet-like article is a composite articlecomprising:(1) a polytetra fluoroethylene fibril matrix, and (2)non-swellable sorptive particles enmeshed in said matrix, the ratio ofnon-swellable particles to PTFE being in the range of 19:1 to 4:1 byweight, said composite article having a net surface energy in the rangeof 20 to 300 milliNewtons per meter, (b) wicking solvent onto saidchromatographic sheet-like article, just inside said spotted circularconfiguration, while said article is continuously rotating, to effectdifferential migration of said components and their separation.
 2. Themethod according to claim 1 further comprising the step of individuallycollecting said resulting separated components.
 3. The method accordingto claim 1 wherein said matrix further comprises in the range of morethan zero and up to 28.99 parts per part of PTFE of non-swellableproperty modifying particles.
 4. The method according to claim 3 whereinsaid consisting of calcium carbonate, ammonium carbonate, kaolin, sugar,polyethylenes, polypropylenes, polyesters, polyamides, polyurethanes,polycarbonates, zeolites, chitosan, glass beads, vermiculite, clay,ceramics, coloring or fluorescing particles, pH modifiers, and particlescoated with these substances.
 5. The method according to claim 1 furthercomprising water-swellable property modifying particles in an amount upto 30 weight percent of the total particles.
 6. The method according toclaim 5 wherein said water-swellable property modifying particles areselected from the group consisting of starch, modified starch, agarose,polyacrylamides, ion exchange and chelating particles, cellulosics,polymethacrylates, styrene-divinylbenzene copolymers, and chitosan. 7.The method according to claim 1 wherein said non-swellable particles areinorganic oxide.
 8. The method according to claim 1 wherein saidnon-swellable particles are an organic compound.
 9. The method accordingto claim 1 wherein said non-swellable particles are a polymer.
 10. Themethod according to claim 1 wherein said non-swellable particles are atleast one of silica and zirconia.
 11. The method according to claim 1wherein said non-swellable particles are silica.
 12. The methodaccording to claim 1 wherein said non-swellable particles are chelatingor ion exchange particles.
 13. The method according to claim 1 whereinsaid particles further comprise a layer of covalently bonded organiccoating.
 14. The method according to claim 1 wherein said particlesfurther comprise a coating of non-swellable polymer.
 15. The methodaccording to claim 1 wherein said chromatographic article has beensubjected to electron beam treatment.
 16. The method according to claim1 wherein said non-swellable particles are hydroxylapatite.
 17. A methodof chromatographic separation comprising the steps of:(a) depositing asolution containing separable components in a circular configurationonto a rotatable chromatographic sheet-like article to provide acircular zone of deposited solution, wherein said chromatographicsheet-like article is a composite article comprising:(1) apolytetrafluoroethylene fibril matrix, and (2) non-swellable sorptiveparticles enmeshed in said matrix, the ratio of non-swellable particlesto PTFE being in the range of 19:1 to 4:1 by weight, said compositearticle having a net surface energy in the range of 20 to 300milliNewtons per meter, (b) wicking solvent onto said chromatographicsheet-like article, just inside said circular zone, while said articleis continuously rotating, to effect differential migration of saidcomponents and their separation.
 18. The method according to claim 17further comprising the step of individually collecting said resultingseparated components.
 19. The method according to claim 17 wherein saidmatrix further comprises in the range of more than zero and up to b28.99 parts per part of PTFE of non-swellable property modifyingparticles.
 20. The method according to claim 19, wherein said propertymodifying particles are selected from the group consisting of calciumcarbonate, ammonium carbonate, kaolin, sugar, polyethylenes,polypropylenes, polyesters, polyamides, polyurethanes, polycarbonates,zeolites, chitosan, glass beads, vermiculite, clay, ceramics, coloringor fluorescing particles, pH modifiers, and particles coated with thesesubstances.
 21. The method according to claim 17 further comprisingwater-swellable property modifying particles in an amount up to 30weight percent of the total particles.
 22. The method according to claim21 wherein said water-swellable property modifying particles areselected from the group consisting of starch, modified starch, agarose,polyacrylamides, ion exchange and chelating particles, cellulosics,polymethacrylates, styrene-divinylbenzene copolymers, and chitosan. 23.The method according to claim 17 wherein said non-swellable particlesare inorganic oxide.
 24. The method according to claim 17 wherein saidnon-swellable particles are an organic compound.
 25. The methodaccording to claim 17 wherein said non-swellable particles are apolymer.
 26. The method according to claim 17 wherein said non-swellableparticles are at least one of silica and zirconia.
 27. The methodaccording to claim 17 wherein said non-swellable particles are silica.28. The method according to claim 17 wherein said non-swellableparticles are chelating or ion exchange particles.
 29. The methodaccording to claim 17 wherein said particles further comprise a layer ofcovalently bonded organic coating.
 30. The method according to claim 17wherein said particles further comprise a coating of non-swellablepolymer.
 31. The method according to claim 17 where said chromatographicarticle has been subjected to electron beam treatment.
 32. The methodaccording to claim 17, wherein said non-swellable particles arehydroxylapatite.